In a case where inverters are operated in synchronization with a commercial power source or where a plurality of inverters are operated in parallel, it is required to detect the phase difference and control the output frequency of the inverters for about several % in order to align the output frequencies and the output voltage phases to each other. For the above purpose, an oscillator for use in the inverters of this type has to be equipped with a frequency-varying function. Further, since the oscillator is operated alone in a case where the commercial power source is interrupted or the like, the accuracy for the output frequency of the device is determined by the accuracy for the frequency from the oscillator.
Generally, since an extremely high accuracy is required for the output frequency in the case of a constant voltage constant frequency inverter, fluctuations including temperature changes and aging changes have to be restricted to less than 0.1%. Thus, it is required for the oscillator that, if the frequency control value is zero or a certain value, the frequency accuracy has to determine the value.
In addition, since the constant voltage constant frequency inverter is often used in a not-interrupted type power source system, it is required that the inverter is highly reliable and stable, includes less number of components and less number of controlling factors.
A circuit structure as shown in FIG. 1 has been known as a variable-frequency oscillator of a high stability using a crystal oscillator. In the circuit shown in FIG. 1, the output frequency Fco from a crystal oscillator 1 is converted into a voltage Vco in a frequency-voltage converter 2 and then introduced into an adder 3. While on the other hand, a control voltge Vc is also introduced to the adder 3 and the input value Vco and the control voltage Vc form a setting value for determining the output frequency. While on the other hand, the output from the adder 3 is supplied by way of a PI controller 4 to a voltage-frequency converter 5 and converted into a pulse signal having frequency N.times.Fo. The signal is frequency-divided to 1/N in a frequency divider 7 into an output signal having frequency Fo and, at the same time, converted by a frequency-voltage converter 6 into a voltage VN.times.Fo and fed back to the adder 3. In such a structure, the output from the adder 3 is the difference between Vc+Vco and VN.times.Fo. By controlling the difference to zero in the PI controller, the output frequency N.times.Fo from the voltage-frequency converter 5 is described as: Fco+(frequency Fc determined by control voltage Vc), providing that the gain and the linearity of the frequency-voltage converters 5 and 6 are same.
In this system, the accuracy for the output frequency Fo is determined by the accuracy for the frequency-voltage converters 2 and 6. Accordingly, it is required for the frequency-voltage converters 2 and 6 that they have the same gain and the linearity as well as highly accurate and stable. However, since the frequency-voltage converter is generally composed by using resistors, capacitors, diodes, differential amplifiers and the likes, the circuit structure is complicated very much, as well as a lot of controlling sections are required, to bring about problems in view of the reliability and stability and cause great troubles in the size of the apparatus and the cost, in order to satisfy the foregoing performances.
Further, since the output voltage N.times.Fo from the voltage-frequency converter 5 is used while divided by the frequency divider into a lower frequency, a voltage-frequency converter of a frequency as high as several MHz is required for the digital control or the like, thereby making the device very much expensive.